In cardiac interventions such as mitral valve repair, aortic valve repair or the closure of septal defects, three-dimensional transesophageal echo (TEE) volume imaging is used together with two-dimensional x-ray projection imaging because different structures are visible in the resulting three-dimensional TEE volume image and two-dimensional x-ray projection image. The three-dimensional TEE volume image and the two-dimensional x-ray projection image, which are visualized side-by-side on a display unit, often correspond to different viewing geometries, which makes a comparison of the two different images difficult.